Log₃2=a 1/log₂3=a log₂3=1/a
log₀,₅81=2*log₂3⁴=8*log₂3=8/a.
1. a11=a1+10d.
a11=-3+7=4.
2. a20=a1+19d.
a20=18-19•0.5=8.5
3. a5=a1+4d.
a5=20+4•3=32.
4. a21=a1+20d.
a21=5.8-20•1.5=-25.8
= (6,25a^2*y^4 + 10ay^3 + 4y^2) + 18,75a^2*y^4 = (2,5ay^2+2y)+18,75a^2*y^4